Present infrared detection systems for the acquisition of target positional information may be divided into two general classes: passive and active systems. The most common of these, the passive system, utilizes only the radiation omitted by the target to supply the input signal. The more complicated, active system irradiates the target with an auxiliary source of infrared. An important disadvantage of the active system, from the military point of view, is that it can betray its location to an enemy. Although the present invention is applicable to both systems, the system of the preferred embodiment, herein described, is a passive system.
Passive infrared detection systems comprise, essentially, an optical scanning unit which receives infrared emission from all objects in its field of vision, an infrared detector of the class of materials which produce a small emf proportional to the amount of radiation received, and which includes lead sulphide, indium antimonide and others; an amplifying system including an electrical filter; and a visual display system for positional plotting of the signals. The main disadvantage with present systems is the difficulty experienced by the operator in discriminating between signals generated by a genuine target such as an aircraft, and spurious signals, generated by background "noise", which clutter the display screen. Such "noise" is produced by emission from matter in suspension in the atmosphere such as clouds and the like which reflect sunlight, as well as from many different man-made sources near the horizon.